1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus, such as a printer or a display apparatus, in which a developing agent, such as a toner, is supplied between a plurality of recording electrodes and a recording medium while a signal voltage is applied to the recording electrodes so as to attach the toner to the recording medium.
2. Description of the Related Art
The aforementioned type of conventional image forming apparatus employs the image forming method disclosed in Japanese Patent Publication No. Sho 51-46707 (which corresponds to U.S. Pat. No. 3,914,771), and in U.S. Pat. Nos. 4,831,394, 4,910,538, 4,943,819 and 5,001,501. FIG. 2 shows an image forming apparatus which employs the above method and which is available on the market.
In FIG. 2, reference numeral 1 denotes recording electrodes; 2, a conductive magnetic toner; 3, a recording medium consisting of a surface layer 3a, a resistance layer 3b, a conductive layer 3c and a support layer 3d; 4, a plastic member; 5, a control circuit substrate; 6, a cleaning brush; 7, a rotary magnet; 8, a non-magnetic cylinder; and 9, a recording medium conveying roller. In the apparatus shown in FIG. 2, the conductive magnetic toner 2, which is a coloring substance, is conveyed over the non-magnetic cylinder 8 by the rotation of the rotary magnet 7, and is thereby supplied to a position overlying the recording electrodes 1. An electric charge (10.sup.-8 to 10.sup.-7 coulomb) is induced in the toner 2 by the application of a voltage corresponding to an image signal generated by the circuit substrate 5 between the conductive layer 3c of the recording medium 3 and the recording electrodes 1, by which the toner 2 is electrostatically attached to the recording medium 3 (1000 to 10000 pF/cm.sup.2) and an image is thereby formed. The endless belt-like recording medium on which the image has been formed makes one revolution, during which the cleaning brush 6, which utilizes volume resistance, scrapes the toner on the recording medium over the recording electrodes 1 located just below the brush 6 so that the toner can be used again.
In the aforementioned conventional image forming apparatus, a plastic forming resin is used to reduce the weight of the apparatus and production cost, and the member made of this plastic forming resin is provided near the surface of the recording medium (at a distance of 2 mm to 19 mm) in order to achieve reduction in the size of the apparatus. This approach causes the following problems.
Generally, plastic members are readily charged unless a special measure is taken to prevent charging. Normally, the plastic member has a surface potential between .+-. several tens of volts and .+-. several hundreds of volts. External influences, such as rubbing, can increase this potential to .+-. several kilo-volts.
The conventional image forming apparatus shown in FIG. 2 is characterized by requiring a recording voltage which is an order of magnitude less than that required by another recording means which employs a toner, such as an electrophotographic recording apparatus, i.e., the apparatus of FIG. 2 requires a voltage of several tens of volts. Therefore, the surface potential of the recording member must be more strictly controlled so that the recording medium has a desired potential over the entire surface thereof uniformly.
However, the use of the charged plastic member near the surface of the recording medium causes distribution of the potential on the surface of the recording medium to vary with time. This change in the distribution of the potential affects images adversely. The mechanism of generating changes in the potential distribution is estimated as follows.
FIG. 3 schematically illustrates the effect of an electric field generated between the recording medium 3 and the plastic member 4. The negative charge in the plastic member 4 generates an electric field between the plastic member 4 and the grounded conductive layer 3c in the recording medium in the direction indicated by the longer arrows. The generated electric field induces positive charges in the conductive layer 3c, and the induced positive charges move through the resistance layer toward the surface layer over a finite time in the manner indicated by the shorter arrows and are trapped in the interface between the resistance layer and the surface layer 3a. The trapped positive charges are not easily cancelled: they induce negative charges in the electrodes when 0 volt (representative of a white image signal) is applied to the recording electrodes during a subsequent recording, by which Coulomb's attractive force is generated and toner, which would not be otherwise attached, is attached to the recording medium. Consequently, unnecessary toner is attached to the white area of the recording medium where no toner image should be formed, and a phenomenon which deteriorates the image quality (hereinafter referred to as fog), thus occurs.